Several techniques are known at present for restoring a "normal" lumbar lordosis in this way, by implanting either a graft which in time fuses the vertebrae together, or a prosthesis which fixes them together immediately, while still also making it possible in time to achieve fusion between the vertebra.
In the second above-mentioned technique, use is made mainly of implants, also known as "cages", some of which are hollow, rigid, and one-piece, with inside/outside intercommunication slots for receiving a bone graft which, via said slots, subsequently fuses with the adjacent vertebrae on either side: in this field, reference can be made to patent application WO 96/08205 published on Mar. 21, 1996 for a "Intervertebral fusion cage of conical shape" and application EP 637 440 published on Feb. 8, 1995 for an "Intersomatic implant for the spinal column". Nevertheless, cages of those types are of outside dimensions that are given and fixed, whereas the distances between pairs of vertebrae are not constant. In addition, the inclinations of the facing vertebral faces to which a given angular position is to be imparted do not enable rigid cages to be used from a posterior direction: they can be inserted only from an anterior direction.
As a result, other types of cage have been developed with two substantially parallel branches connected to a rigid body through which it is possible to turn a wormscrew system which then moves a wedge in screw engagement on said screw from an initial position close to the distal ends of the branches towards the body linking the branches together, thereby splaying the two branches apart angularly. It is then possible to insert such a cage of initially flat shape between the vertebrae, and then by turning the drive axis of the wedge, the desired angle between the branches is adjusted or set from a posterior access.
Such cages or implants are described, for example, in European patent application EP 664 994 published on Aug. 2, 1995, entitled "Vertebral intersomatic cage" or in application EP 2 719 763 published on Nov. 17, 1995, and entitled "Vertebral implant".
Nevertheless, such devices which are more mechanical than hollow and rigid cages, and therefore more complex, leave a smaller inside volume for the fusion graft, and because of their flat shape which is not circularly symmetrical, even though they are better at ensuring a given bearing angle between the vertebrae, they require a passage of the same rectangular section to be prepared to receive them, and that complicates implementation.